In order to increase the hardness of steel, so-called quenching treatment in which steel is heated up to a predetermined temperature and thereafter is cooled is generally performed. Specifically, first, steel is heated up to a temperature between 911° C. and 1392° C. under 1 atmospheric pressure, and thereby the phase of the steel is changed into austenite. Subsequently, the steel of austenite is quenched, and the phase thereof is changed into martensite. In this way, the hardness of steel is increased.
When heat treatment such as quenching is performed, a vacuum heat treatment device is used. The vacuum heat treatment device has a double structure including a vacuum furnace and a heat-insulating container which is provided inside the vacuum furnace, wherein a treatment object is disposed inside the heat-insulating container. When the heat treatment using the vacuum heat treatment device is performed, first, the vacuum furnace and the heat-insulating container are opened and a treatment object is disposed inside the heat-insulating container, and subsequently, the vacuum furnace is closed and the inside thereof is made into a vacuum state. When the inside reaches the vacuum state, the heat-insulating container is closed and the treatment object is heated. After a predetermined period has passed, the heat-insulating container is opened, a cooling medium is supplied into the vacuum furnace and into the heat-insulating container, and the treatment object disposed in the heat-insulating container is cooled.
In a case where a treatment object is cooled at the vacuum heat treatment device as described above, a cooling medium is supplied into the heat-insulating container. At this time, if the cooling medium does not reach every part inside the heat-insulating container, the cooling to the treatment object may not be uniformly performed. For example, when the heat treatment of steel as a treatment object is performed, the unevenness in quenching may occur, and the hardness of steel may become non-uniform. In addition, when the heat treatment of stainless steel as a treatment object is performed, sensitization may occur.
In order to cool a treatment object, a vacuum heat treatment device is disclosed in which nitrogen gas is supplied into a vacuum furnace, a fan circulates the nitrogen gas in a heat-insulating container, and the nitrogen gas discharged from the heat-insulating container is cooled and is supplied into the heat-insulating container again (e.g., refer to Patent Document 1). In the vacuum heat treatment device disclosed in Patent Document 1, in order to efficiently guide nitrogen gas into the heat-insulating container, wind direction guide vanes are fixed to an inner wall of the vacuum furnace.